This doesn't directly answer your question, but there are 2 ways an employer can contribute to an HSA, either as a pre-tax withholding that gets reported in Box 15 under code W , or, through a Section 125 Cafeteria plan. This type of contribution withholds the amount from your box-1 Wages, so it lowers that number by the contribution amount of your pre-tax withholding and the employer contribution to the HSA.
I just went through this, because my employer reports it in box 15, but my wife's is a sec 125 plan. When a sec 125 plan, you can't count that as a 'contribution' because it's already lowered your box-1 wages.
Sorry, but this information is incorrect. Please see the language from W-2 and W-3 Instructions below.
Health savings account (HSA). An employer's contribution (including an employee's contributions through a cafeteria plan) to an employee's HSA is not subject to federal income tax withholding or social security, Medicare, or railroad retirement taxes (or FUTA tax) if it is reasonable to believe at the time of the payment that the contribution will be excludable from the employee's income. However, if it is not reasonable to believe at the time of payment that the contribution will be excludable from the employee's income, employer contributions are subject to federal income tax withholding, social security and Medicare taxes (or railroad retirement taxes, if applicable), and FUTA tax, and must be reported in boxes 1, 3, and 5 (use box 14 if railroad retirement taxes apply), and on Form 940, Employer's Annual Federal Unemployment (FUTA) Tax Return.
You must report all employer contributions (including an employee's contributions through a cafeteria plan) to an HSA in box 12 of Form W-2 with code W. Employer contributions to an HSA that are not excludable from the income of the employee also must be reported in boxes 1, 3, and 5. (Use box 14 if railroad retirement taxes apply.) An employee's contributions to an HSA (unless made through a cafeteria plan) are includible in income as wages and are subject to federal income tax withholding and social security and Medicare taxes (or railroad retirement taxes, if applicable). Employee contributions are deductible, within limits, on the employee's Form 1040. For more information about HSAs, see Notice 2004-2, Notice 2004-50, and Notice 2008-52. Notice 2004-2, 2004-2 I.R.B. 269, is available at IRS.gov/irb/ 2004-02_IRB/ar09.html. Notice 2004-50, 2004-33 I.R.B. 196, is available at IRS.gov/irb/2004-33_IRB/ar08.html. Notice 2008-52, 2008-25 I.R.B. 1166, is available at IRS.gov/irb/2008-25_IRB/ar10.html. Also see Form 8889, Health Savings Accounts (HSAs), and Pub. 969.
https://www.irs.gov/pub/irs-pdf/iw2w3.pdfSee also Instructions from 8889
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Employer Contributions Employer contributions (including contributions through a cafeteria plan) include any amount an employer contributes to any HSA for you for 2016. These contributions should be shown in box 12 of Form W-2 with code W. If either of the following apply, complete the Employer Contribution Worksheet. Employer contributions for 2015 are included in the amount reported in box 12 of Form W-2 with code W. Employer contributions for 2016 are made in 2017. If your employer made excess contributions, you may have to report the excess as income. See Excess Employer Contributions, later.
https://www.irs.gov/pub/irs-pdf/i8889.pdf